(1) Field of the Invention
The present invention relates to a wavelength division multiplexing (WDM) optical transmission system and a WDM optical transmission method, for multiplexing a plurality of signal light with different wavelengths and transmitting them in an identical optical fiber to realize a large capacity communication. In particular, the present invention relates to a technique for multiplexing each signal light at a high density for transmission.
(2) Description of Related Art
In order to increase transmission capacity of a WDM optical transmission system, it is necessary to reduce frequency spacing (wavelength spacing) as narrower as possible, and to multiplex many wavelengths at a high density. However, a spectrum or signal light has width depending on a bit rate of the signal light, and the spectrum width limits the frequency spacing.
The abovementioned signal light spectrum width depends not only on the bit rate but also on a modulation and demodulation system. Following systems are known, for example, as modulation and demodulation systems used for the WDM optical transmission system.                (1) Intensity Modulation—Direct Detection (IM-DD) System using an NRZ Modulation Type        (2) Intensity Modulation—Direct Detection (IM-DD) system using an RZ Modulation Type        (3) CS-RZ (Carrier Suppressed-R7) Modulation—direct Detection System (for example, refer to “1.2-Tbit/s (30×42.7-Gbit/s ETDM optical channel) WDM transmission over 376 km with 125-km spacing using forward error correction and carrier-suppressed RZ format”, by Y. Miyamoto, OFC2000 PD26, and the like)        (4) BSIM-DPSK Modulation—Direct Detection System (for example, refer to “Suppression of degradation induced by SPM/XPM+GVD in WDM transmission using a bi-synchronous intensity modulated DPSK signal”, by T. Miyano, OECC2000 14D3-3, and the like)        (5) VSB Modulation—Direct Detection System (for example, refer to “Study on “20 Gbit/s WDM transmission by band reduction RZ optical signal using optical filter”, by T. Tsuritani, OCS2001-28, and the like)        
Among these modulation systems, the system (1) is the most widely used for actual products. The systems (2) to (4) each have an advantage of a higher resistance to OSNR than the system (1). However, since the signal light has the wider spectrum, there is a disadvantage from the viewpoint of high density multiplexing. Further, the system (5) has a narrower signal light spectrum than the system (1), which is advantageous from the viewpoint of high density multiplexing, but has a disadvantage in that a constitution of optical sender is complicated.
Moreover, as a measure for making the WDM signal light to be at a high density, for example, a technique based on the following polarization control is proposed in addition to the techniques described above centered on the modulation and demodulation system.                (6) Orthogonal Polarization Transmission Technique (refer to “6.4 Tb/s (160×40 Gb/s) WDM Transmission Experiment with 0.8 bit/s/Hz Spectral Efficiency”, by T. Ito, ECOC2000 PD1.1, and the like)        (7) Polarization Division Multiplexing Transmission Technique (for example, refer to “Transmission of 256 wavelength-division and polarization-division-multiplexed channels at 42.7 Gb/s (10.2 Tb/s capacity) over 3×100 km of TeraLight™ fiber”, by Y. Frignac, OFC2002 Post Deadline Papers FC5-1, and the like)        
Incidentally, spectrum efficiency is known as an index for representing the high density of WDM signal light. This spectrum efficiency is defined by a value (B/S) obtained by dividing a bit rate B per one wave by frequency spacing S.
For example, in the intensity modulation-direct detection (IM-DD) system using a normal NRZ modulation type as described in (1), even in the case where neither the orthogonal polarization transmission technique as described in (6) nor the polarization division multiplexing transmission technique as described in (7) is used, the maximum spectrum efficiency of 0.4 bit/s/Hz is achieved. To be specific, there are reported a case where signal light of 10 Gbit/s per one wave is multiplexed at 25 GHz spacing (for example, refer to “25 GHz spaced DWDM 160×10.66 Gbit/s (1.6 Tbit/s) Unrepeatered Transmission over 30 km”, by P. Le Roux, ECOC2001 PDM1.5, and the like), or a case where signal light of 40 Gbit/s per one wave is multiplexed at 100 GHz spacing (for example, refer to “3.5 Tbit/s (43 Gbit/s×88 ch) transmission over 600-km NZDSF with VIPA variable dispersion compensators”, by H. Ooi, OFC2002 ThX3, and the like).
Further, at a research level, the spectrum efficiency of over 0.4 bit/s/Hz has been realized by applying techniques regarding polarization control as described in (6) and (7) to the VSB modulation-direct detection system as described in (5).
However, in order to realize the aforementioned orthogonal polarization transmission technique and polarization division multiplexing transmission technique, since assembly becomes very complicated as the number of parts in an optical sender and an optical receiver increase, there is a problem in that size and cost are increased. Therefore, it is required to realize the spectrum efficiency of over 0.4 bit/s/Hz without performing orthogonal polarization transmission and polarization division multiplexing transmission by a constitution using a small sized and low cost optical sender and optical receiver, to achieve a large capacity system.
A major problem in achieving an increase in spectrum efficiency is that the Q-value is degraded due to cross-talk between optical signals. That is, even though the spectrum efficiency can be increased to expand the transmission capacity, if as a result, the Q-value is degraded and a transmission distance shortened, there is a case where market demands cannot be satisfied.
In order to discuss system performance from such a viewpoint, it is effective to use not only the spectrum efficiency but also, for example, the product of transmission distance, and transmission capacity (hereunder referred to as transmission distance-capacity product) as performance indexes, and in the system designing, maximization of the above-described transmission distance-capacity product is an important task. In order to maximize the transmission distance-capacity product, it is important to suppress the Q-value degradation due to cross-talk between optical signals.